Travel Demand Model - OKI

OKI/MVRPC Travel Demand Model – Version 6.02.2 Model Calibration ProcessThe OKI/MVRPC model uses logsums as the impedance for HBW, HBU, HBO and NHB peak andoff peak trip distribution models. Because the logsums are obtained from the mode choicemodel, full model calibration requires iteration between the calibration of the gravity models andthe calibration of the mode choice models.For the initial trip distribution calibration, the mode choice model was applied using thealternative-specific constants obtained in model estimation. After the first calibration, the tripdistribution model is applied, and the resulting trip tables are used to re-calibrate the modechoice model. With the newly computed alternative-specific constants, the calibration of thegravity models is checked again, and adjusted if necessary. This process of iterating betweenthe mode choice calibration and the trip distribution calibration is repeated until there are nochanges in the convergence criteria of the calibration process. The entire calibration process wasrepeated several times during the model validation phase to incorporate any changes in logsumsdue to refinements made to either the mode choice or the trip distribution models during thisphase. The results reported below correspond to the final trip distribution and mode choicecalibrations.2.3 Model Calibration DataThe calibration of gravity models requires three data items: the impedance matrix, an observedtrip table, and observed trip production and attractions. Below is a description of these dataitems.2.3.1 Impedance Matrix (Logsums)The logsums are obtained from a special application of the mode choice model. The controlparameters of the mode choice application program allow the user to output either trip tables orlogsum tables. Because the computation of logsums requires only the calculation of mode choiceprobabilities, it is not necessary to have a trip table in order to produce logsums.2.3.2 Observed Trip DataThe observed trip table and observed productions and attractions were developed from the OKIhome interview survey, based on expanded data records. It was not possible to includeinformation about MVRPC trips on the trip distribution calibration because the MVRPC survey isnot geocodable. It must be assumed then that the OKI Council region trip length pattern appliesto MVRPC. This assumption and possibly the trip distribution calibration should be revised asMVRPC origin/destination data become available. A correction to the gravity model, in the formof K-factors for trip interchanges between OKI and MVRPC, was computed based on data fromthe ODOT cordon surveys. This process is described in Section 2.62.4 Friction Factor CalculationThe purpose of the calibration process is to determine a set of friction factors that, when appliedin the gravity model, yield an estimated trip length frequency distribution (TLFD) function withcharacteristics similar to the observed TLFD function.Trip Distribution - Gravity Model Calibration 3